feat: require school selection before email verification

[SonJH7]

Summary

• add the selectSchool API to register a user’s school prior to email verification
• require users to choose Busan National University before entering an email and disable certification controls until selection is complete
• refresh related styles and tests to cover the new school-selection flow

Testing

• pnpm lint
• pnpm vitest run src/tests/certification/EmailCertPage.test.tsx src/tests/routing/routeGuards.test.tsx

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https://chatgpt.com/codex/tasks/task_e_690ce0d3fb0c8332bb587489a626c6de